This invention relates generally to an apparatus for the logging of earth boreholes and specifically to a well logging device having one or more pads adapted to be urged against the borehole wall.
It is well known in the art of well logging to utilize apparatus having an elongated body adapted to traverse an earth borehole and also having one or more pad devices extendible from the main body into engagement with the borehole wall. When using such devices, pressure frequently builds up between the face of the measuring pad, or the back-up pad, and the borehole wall which can seriously damage the measuring device. Those in the art, while recognizing that such a differential pressure exists, have concentrated their efforts on controlling the force with which the pad is pushed up against the wall. Not infrequently, this has resulted in inferior measurements made with the well logging instrument.
Furthermore, it is known in the above-noted related application to provide one or more pressure relief holes in such pad devices. However, in such devices, the face of the pad is in continuous contact with the borehole fluid.
It is therefore the primary object of the present invention to provide a new and improved apparatus for logging earth boreholes with apparatus of the type using pad devices which are provided with means selectively operable for equalizing the pressure across the pad; and
It is also an object of the present invention to provide a new and improved well logging apparatus having one or more pad assemblies in which such pads have been perforated in a manner which tends to minimize the effect of the holes on the focusing properties of the electrode arrangement.
The objects of the invention are accomplished, generally, by a well logging apparatus having one or more pad assemblies thereon adapted for urging the same against the borehole wall, such pads being characterized by one or more selectively opening pressure relief holes therein which tend to eliminate the buildup of a differential pressure across the pad.